Add example for caching.

This not only helps try out library but also encourages future contribution to issue dotnet#67770.

src/libraries/Microsoft.Extensions.Caching.Memory/README.md

@@ -4,6 +4,40 @@ In-memory caching provides a general purpose memory implementation of the core c
 
 Documentation can be found at https://learn.microsoft.com/en-us/dotnet/core/extensions/caching.
 
+## Example
+
+The following example shows how to instantiate a single memory cache using `AddMemoryCache` API and via DI get it injected to enable them calling `GetCurrentStatistics`
+
+```c#
+// when using `services.AddMemoryCache(options => options.TrackStatistics = true);` to instantiate
+
+    [EventSource(Name = "Microsoft-Extensions-Caching-Memory")]
+    internal sealed class CachingEventSource : EventSource
+    {
+        public CachingEventSource(IMemoryCache memoryCache) { _memoryCache = memoryCache; }
+        protected override void OnEventCommand(EventCommandEventArgs command)
+        {
+            if (command.Command == EventCommand.Enable)
+            {
+                if (_cacheHitsCounter == null)
+                {
+                    _cacheHitsCounter = new PollingCounter("cache-hits", this, () =>
+                        _memoryCache.GetCurrentStatistics().CacheHits)
+                    {
+                        DisplayName = "Cache hits",
+                    };
+                }
+            }
+        }
+    }
+```
+
+The stats can be viewed using the above event counter like below with `dotnet-counters` tool:
+
+<img width="400" alt="image" src="https://user-images.githubusercontent.com/5897654/156053460-46db5070-04b0-478c-9013-ab0298a7b1ec.png">
+
+To learn more about using memory cache and getting statistics check out issue dotnet/runtime#67770.
+
 ## Contribution Bar
 - [x] [We consider new features, new APIs, bug fixes, and performance changes](../../libraries/README.md#primary-bar)